Voiceband communication channels or facilities are now commonly used to transmit other than voice signals. Of particular importance is the transmission of data signals. Consequently, it is desirable to obtain simply and quickly a measure of parameters which affect the transmission of data signals over voiceband communications facilities.
One such parameter is commonly known as nonlinear distortion. As described in the Bell System Technical Reference, PUB41008, entitled "Analog Parameters Affecting Voiceband Data Transmission-Description of Parameters," dated Oct. 21, 1971, nonlinear distortion was first measured by applying a single frequency tone to a transmission facility and measuring so-called second and third order distortion products with a frequency selective voltmeter or a spectrum analyzer. However, it was found that the use of a single frequency test signal does not properly characterize the nonlinear distortion of most telecommunications facilities.
The above-noted PUB41008 also describes a technique for obtaining a measure of nonlinear distortion which utilizes a multitone test signal. For a multitone test signal, the nonlinear distortion is termed intermodulation distortion and appears as tones at frequencies which are linear combinations of the frequencies of the tones in the test signal. One example is given of a test signal consisting of three tones having equal amplitudes at three arbitrary frequencies. One problem with the use of such a multitone test signal is that the distortion products to be measured appear at a number of frequencies in the frequency band of interest. Consequently, it becomes quite burdensome to obtain a measurement since the products at all the frequencies must be determined. Moreover, use of three arbitrary tones does not yield a probability density function which properly characterizes the nonlinear distortion of communication facilities encountered in telecommunication systems.
As indicated in the Bell System Technical Reference, PUB41009, entitled "Transmission Parameters Affecting Voiceband Data Transmission-Measuring Techniques," dated January 1972, proper characterization of intermodulation distortion of a communications facility may be realized by employing a test signal including first and second narrow bands of Gaussian noise centered about a first prescribed frequency and a second prescribed frequency, respectively. This test signal is then applied to a communication facility under evaluation and the second and third order intermodulation distortion products are measured. Use of the noise bands causes the distortion products to be in prescribed frequency bands, thereby simplifying the measurement. One problem, however, with the use of noise bands in the test signal is the relatively long interval, approximately 30 seconds to one minute, required for each measurement to stabilize.
One attempt at overcoming the problems of using noise bands in the test signal employs a technique which uses four equal amplitude tones. The equal amplitude tones are arranged in first and second pairs with a predetermined frequency separation between the tones in each pair. The tone pairs are employed to simulate the noise bands used in the technique discussed in the above-noted PUB41009. The first and second tone pairs are then centered about first and second prescribed frequencies, respectively. The four-tone test signal is then applied to a facility under evaluation and second and third order intermodulation distortion products are again measured. One arrangement employing four equal amplitude tones to simulate noise bands in intermodulation distortion measurements is described in U.S. Pat. No. 3,862,380 issued to Norris C. Hekimian, et al., on Jan. 21, 1975.
It has been recognized that use of a test signal including four equal amplitude tones to obtain a measure of intermodulation distortion results in inaccuracies when applied to certain transmission facilities. In particular, problems result when applying the four-tone test signal to transmission facilities including companders. Apparently, companders tend to track slow, i.e., long term variations in the envelope of a signal being transmitted over the facility. This is referred to as tracking the syllabic rate of the transmitted signal, i.e., tracking the long term amplitude variations in the envelope of the input signal supplied to the compander. When the prior four-tone test signal is used on such transmission facilities, the so-called syllabic rate is in a frequency range that causes unwanted variations in the output signal from the compander. It has been determined that the output variations caused by the four-tone test signal are different from those caused by an actual transmitted data signal. Consequently, unwanted variations are caused in the measurement of intermodulation distortion on that facility, thereby yielding erroneous results.
Additionally, it has become increasingly desirable to employ digital techniques in the acquisition of measurement data. It has also been determined that the use of the prior four-tone test signal in measurement apparatus employing digital data acquisition requires the use of memory units having more capacity than desired. Indeed, it is desirable to employ memory units having a reasonable block size (512 words as compared to 4096) for generating the test signals and for processing signals. When using smaller memory block size and attempting to use a four tone test signal, wider frequency spacing of the intermodulation distortion products results. Consequently, the measurements are more susceptible to noise signals and the like on the transmission facility, again yielding erroneous measurement results.
Moreover, the intermodulation distortion products to be measured occur in frequency bands where background noise or other unwanted signal components (for example, quantizing noise, and the like) usually are found. Consequently, measurements employing prior apparatus include an error component proportional to the noise power in the frequency bands of interest. This error component is compensated for when using prior measurement apparatus by an operator either employing a prepared noise power chart or making a separate noise measurement and then mentally subtracting the noise power value from the measured intermodulation distortion products values.